1. Field of the Invention
The present invention relates to a projection lens, and more particularly to a projection lens which is bright and has a high resolution suitable for a projection lens of a high definition television.
2. Description of the Prior Art
As is known, a television projector is to project an display plane of a cathode-ray tube or a liquid crystal display plane on a screen in an enlarged scale by a projection lens. The television projector now has a demand not only for business but also for home. On the other hand, a so-called high definition television having a resolution about twice of a conventional television has a sign for a practical use mainly for business use and is expected to spread for home use. Under these situations, a television projector also has a higher demand for its high image quality. A television projector which can maintain a highly precise and fine image quality as seen in a high definition television has been required.
For realizing the aforementioned desire, an attempt has been made not only to improve a lens itself but curve an display plane of a cathode-ray tube and an display plane of a liquid crystal so as to obtain a high performance. These examples are disclosed in Japanese Patent Application Laid-Open Publication Nos. 181009/1983, 200215/1985, 145517/1986, 264716/1988, etc. In these publications, monochromatic aberrations are corrected but chromatic aberrations are not sufficiently corrected, being difficult to obtain a highly precise and fine image.
On the other hand, as examples of lenses which enable correction of chromatic aberrations, lenses as described in Japanese Patent Application Laid-Open Publication Publication Nos. 241717/1986, 85515/1988 and the like are known. However, these lenses have not been sufficiently fulfilled with the desire for obtaining a further highly precise and fine image.
Examples of a projector lens intended to obtain a highly precise and fine image quality are seen in Japanese Patent Application Laid-Open Publication Nos. 106427/1987, 85212/1987 and the like. In these lenses, as will be apparent from examples thereof, an aspherical lens is used as a first lens to correct the spherical aberration, the comatic aberration, etc. caused by an aperture.
However, actually, the technique for preparation of an aspherical lens is not-yet completed, and at present the preparation thereof is extremely difficult. In case of an aspherical lens formed of plastics. which is relatively simple in processing, this lens is inferior to a glass lens in environmental characteristics such as heat resistance, humidity resistance and the like. Particularly, in case where an aspherical lens is used in a portion very sensitive to the error of the aberration caused by an aperture as in the first lens, the spherical aberration is much affected, and higher order flare occurs to lower the contrast and deteriorate the image quality, thus making it difficult to realize a highly precise and fine projector.
In order to solve this problem, it is contemplated to use a spherical lens as a first lens. However, it has been heretofore essential to use an aspherical surface for the first lens group in order to realize a highly precise and fine image quality in design. Therefore, it has been contemplated to be difficult to realize it using the spherical lens for the first lens.
However, there is a high desire to the realization of a projection lens for a projector which can well correct various aberrations and can obtain a highly precise image quality as in a high definition television with less components while using a spherical lens as a first lens. The present invention is to provide a projection lens which can satisfy such desires.